Multi-channel communication systems are often susceptible to interference between the various channels, also referred to as crosstalk or inter-channel crosstalk. For example, digital subscriber line (DSL) broadband access systems typically employ discrete multi-tone (DMT) modulation over twisted-pair copper wires. One of the major impairments in such systems is crosstalk between multiple subscriber lines within the same binder or across binders. Thus, signals transmitted over one subscriber line may be coupled into other subscriber lines, leading to interference that can degrade the throughput performance of the system. More generally, a given “victim” channel may experience crosstalk from multiple “disturber” channels, again leading to undesirable interference.
Different techniques have been developed to mitigate crosstalk and to maximize effective throughput, reach and line stability. These techniques are gradually evolving from static or dynamic spectrum management techniques to multi-channel signal coordination.
By way of example, certain of the above-noted techniques allow active cancellation of inter-channel crosstalk through the use of a postcoder. In DSL systems, the use of a postcoder is contemplated to achieve crosstalk cancellation for upstream communications between customer premises equipment (CPE) or other types of network terminals (NTs) and a central office (CO) or another type of access node (AN). It is also possible to implement crosstalk control for downstream communications from the AN to the NTs, using so-called pre-compensation techniques implemented by a precoder.
Crosstalk estimates are commonly utilized in situations in which it is necessary to “join” an additional line to a group of active lines in a DSL system. For example, it may become necessary to activate one or more inactive lines in a synchronization group that already includes multiple active lines, where synchronization in this context refers to alignment in time of the DMT symbols for the different lines. Such joining of an additional line may require that the postcoder be adjusted accordingly in order to optimize system performance. Crosstalk estimates are also used in other situations, such as tracking changes in crosstalk over time. Thus, crosstalk estimation may be used to determine the residual crosstalk after postcoding and this information can be used to adjust the crosstalk coefficients.
Conventional crosstalk reduction techniques are deficient in terms of the information transfer rate required between a given receiver and a postcoder. For example, in certain DSL systems it is known to perform time-domain processing of the received signal, including determining the boundaries of the DMT symbols, followed by a transformation in the frequency domain to enable tone-based processing of the DMT symbols. Each tone that is part of the upstream band contains a received frequency-domain signal that can be represented by a complex value. The real and imaginary components of this value are typically presented by m bits each. In order to perform interference post-compensation, the receiver may supply the set of signals corresponding to the tones belonging to the upstream band to the postcoder using the m-bit representations of the complex values. More specifically, the signals corresponding to the upstream tones are sent by the receiver to the postcoder with m bits being used to represent each of the real and imaginary components of a given signal, such that 2m bits are required to represent each signal for each tone. Such an arrangement unduly increases the bandwidth requirements of the postcoder interface, and limits the throughput performance of the system. Also, the use of the m-bit representation can introduce quantization errors into the signals that are applied to the postcoder.
Accordingly, a need exists for improved post-compensation arrangements that can reduce the bandwidth requirements of the postcoder interface while also limiting the adverse impact of quantization error on received signals.